1. Field of the Invention
The present invention relates to a wavelength-divided multiplexing (WDM) transmission system for optical communication and to a wavelength tunable filter apparatus used for an apparatus of such a system.
2. Related Art of the Invention
In the optical communication field, a wavelength-multiplexed optical transmission method is already known which is used to transmit multi-channel signals on one optical fiber. A wavelength-multiplexed optical transmission method is a method in which an optical wavelength coupler wavelength-multiplexes signals of respective channels with light having different wavelengths from each other, the signals are transmitted through one optical fiber from the sending side to the receiving side, a wavelength filter separates on the receiving side the multiplexed transmission light with respect to the respective wavelengths and the signals are reproduced. For this reason, a wavelength tunable filter is used which can change a wavelength which is to be received.
In this manner, it is possible to freely extract light having a desired wavelength from light which is formed of a number of multiplexed wavelengths. This is the reason that a wavelength tunable filter apparatus is being explored which is addition of a wavelength-tuning function of correctly selecting a received wavelength to a wavelength tunable filter.
FIG. 10 shows a structure of a conventional wavelength tunable filter apparatus and a flow of a data signal in each part. In FIG. 10, denoted at 101 is a wavelength tunable filter, denoted at 102 is a photoelectric conversion part, denoted at 103 is a received light intensity detecting part, denoted at 104 is a wavelength/intensity memory part, denoted at 105 is a peak position detecting part, and denoted at 106 is a wavelength scanning (changing) part, which form the wavelength tunable filter apparatus. Denoted at 107 is an electric signal processing part.
Now, operations of such a conventional apparatus will be described. The photoelectric conversion part 102 converts an optical signal from the wavelength tunable filter 101 into a photoelectrically converted signal. The photoelectrically converted signal is supplied to the wavelength scanning part 107 while partially detected by the received light intensity detecting part 103 to obtain an average value of the photoelectrically converted signal, namely, an average received light intensity. The average received light intensity is stored in the wavelength/intensity memory part 104 together with wavelength position data obtained from the wavelength scanning part 106.
While the wavelength scanning part 106 changes a wavelength of the wavelength tunable filter 101, based on the information stored in the wavelength/intensity memory part 104, the peak position detecting part 105 specifies a wavelength at which the average received light intensity becomes maximum and supplies the position of such a wavelength to the wavelength scanning part 106, whereby the wavelength is set.
On the other hand, another method already proposed (e.g., Japanese patent no. 2,655,479) requires that wavelength-multiplexed light is supplied to a wavelength tunable filter, received light intensities of light having two wavelengths which are adjacent to a wavelength which is to be selected are monitored and a difference between the intensities is optimized.
However, in the conventional wavelength tunable filter apparatus described above, when a received light intensity at the wavelength tunable filter 101 decreases, in order to determine whether the set wavelength of the wavelength tunable filter 101 has shifted toward the short-wavelength side or the long-wavelength side with respect to a transmitted oscillation wavelength of a light source, it is necessary to detect a received light intensity after shifting the set wavelength of the wavelength tunable filter 101 toward the short-wavelength side and the long-wavelength side and to specify a direction which increases a received light intensity.
Thus, the process described above demands to change a wavelength at least twice, and when a wavelength is changed in a direction which decreases a received light intensity, a signal quality may temporarily degrade owing to the decrease in the received light intensity.
Further, the method which requires to monitor an average received light intensity of light having adjacent wavelengths has a problem that it may become impossible to correctly set a wavelength if original intensity levels of the light having adjacent wavelengths change.
Noting the problems as above with such conventional wavelength tunable filter apparatuses, the present invention aims at providing a wavelength tunable filter apparatus which allows to accurately set a wavelength once again when a received light intensity decreases, by means of a simple wavelength change using only light which has a wavelength to be selected.
The 1st invention of the present invention (corresponding to claim 1) is a wavelength tunable filter apparatus, comprising:
a wavelength tunable filter which can change a wavelength to be selected for an inputted optical signal;
a photoelectric conversion part which converts transmitted light received from said wavelength tunable filter into a received light intensity signal;
a received light intensity detecting part which detects an intensity of said received light intensity signal which is converted;
a signal component detecting part which detects a signal component from said received light intensity signal;
a signal intensity detecting part which detects an intensity of said signal component which is detected;
an intensity variation detecting part which detects a variation in said received light intensity detected by said received light intensity detecting part and an intensity variation in said signal component detected by said signal intensity detecting part; and
wavelength setting means which, on the basis of said detected both variations, causes said wavelength tunable filter to change the wavelength to be selected as to set the wavelength appropriately such that an output from said photoelectric conversion part satisfies a predetermined condition.
The 2nd invention of the present invention (corresponding to claim 2) is the wavelength tunable filter apparatus of said the 1st invention, wherein a direction of the changing of the wavelength to be selected by said wavelength tunable filter is determined based on said both variations.
The 3rd invention of the present invention (corresponding to claim 3) is the wavelength tunable filter apparatus of said the 1st invention, wherein the wavelength to be selected by said wavelength tunable filter is changed by a feedback method.
The 4th invention of the present invention (corresponding to claim 4) is the wavelength tunable filter apparatus of said the 1st invention, wherein said wavelength setting means comprises a wavelength/intensity memory part which stores in advance reference data which describe a state of the intensity variation in said received light intensity signal in a direction in which said wavelength tunable filter changes the wavelength and/or a state of the intensity variation in said signal component, and the wavelength to be selected by said wavelength tunable filter is changed in accordance with comparison of said reference data against at least one of said state of the detected variations.
The 5th invention of the present invention (corresponding to claim 5) is the wavelength tunable filter apparatus of said the 1st invention, wherein said wavelength setting means sets as a set wavelength a wavelength which maximizes the intensity of said received light intensity signal.
The 6th invention of the present invention (corresponding to claim 6) is the wavelength tunable filter apparatus of said the 1st invention, wherein said wavelength setting means sets as a set wavelength a wavelength which maximizes the intensity of said signal component.
The 7th invention of the present invention (corresponding to claim 7) is the wavelength tunable filter apparatus of said the 1st invention, wherein said wavelength setting means sets as a set wavelength a wavelength which is between a wavelength which maximizes the intensity of said signal component and a wavelength which maximizes the intensity in said signal component.
The 8th invention of the present invention (corresponding to claim 8) is the wavelength tunable filter apparatus of said the 1st invention, wherein said a difference between said both variations is further detected and said set wavelength is set based on the difference.
The 9th invention of the present invention (corresponding to claim 9) is the wavelength tunable filter apparatus of said the 4th invention, wherein when a relationship between said both two variations is such a relationship which is indicative of predetermined abnormality, said wavelength setting means transmits a signal notifying the abnormality and/or stops wavelength scanning.
The 10th invention of the present invention (corresponding to claim 10) is the wavelength tunable filter apparatus of any one of said the 1st through 9th inventions, wherein an electric signal detected by said signal component detecting part is a signal which is formed by multiplexing some modulated sub-carrier frequency component signals, one of the sub-carrier signals is selectively extracted, and the intensity of the extracted signal component is supplied to said signal intensity detecting part.
The 11th invention of the present invention (corresponding to claim 11) is the wavelength tunable filter apparatus of any one of said the 1st through 9th inventions, wherein an electric signal detected by said signal component detecting part is a signal which is formed by multiplexing some modulated sub-carrier frequency component signals and non-modulated sub-carrier frequency component signal, non-modulated sub-carrier signal is selectively extracted, and the intensity of the extracted signal component is supplied to said signal intensity detecting part.
The 12th invention of the present invention (corresponding to claim 12) is the wavelength tunable filter apparatus of according to said the 1st invention, wherein used as said wavelength tunable filter is a wavelength tunable filter which is formed by any one of a filter using a diffraction grating, a fiber diffraction grating, an interference film filter, a Fabry-Perot etalon filter, an acoustic optic filter, and a Mach-Zehnder filter or a wavelength tunable filter which is formed by a combination of these filters.
The 13th invention of the present invention (corresponding to claim 13) is an optical receiving apparatus which receives at least one wavelength light and comprises:
the wavelength tunable filter apparatus of said the 1st invention; and
a signal outputting part which outputs an electric signal component from said signal component detecting part of said wavelength tunable filter.
The 14th invention of the present invention (corresponding to claim 14) is an optical transmission apparatus, comprising:
an optical transmitting apparatus which comprises one electro-optic conversion part which receives an electric signal and electro-optically converts the electric signal into an optical signal having a predetermined wavelength, or a plurality of electro-optic conversion parts each receiving an electric signal and electro-optically converting the electric signals into optical signals whose wavelengths are different from each other, and an optical coupler part which combines the optical signals outputted from said electro-optic conversion part or parts; and
the optical receiving apparatus of said the 13th invention which directly or indirectly receives an optical signal outputted from said optical transmitting apparatus.
The 15th invention of the present invention (corresponding to claim 15) is an optical transmission system, comprising:
an optical transmitting apparatus which comprises one electro-optic conversion part receives an electric signal and electro-optically converts the electric signal into an optical signal having a predetermined wavelength, or a plurality of electro-optic conversion parts each receiving an electric signal and electro-optically converting the electric signals into optical signals whose wavelengths are different from each other, and an optical coupler part which combines the optical signals outputted from said electro-optic conversion part or parts;
an optical transmission path for transmitting an optical signal outputted from said optical transmitting apparatus; and
the optical receiving apparatus of said the 13th invention which receives an optical signal outputted from said optical transmission path.